Traditionally, WDM (Wavelength Division Multiplexing) is known as a technique for optical multiplex transmission of information. With WDM it is possible to multiplex optical signals with different wavelengths and transmit plural pieces of information by one optical fiber.
Formerly the following relay station using a phase conjugate light was proposed. The relay station is located between a sending station and a receiving station and is connected to them via an optical transmission line. The relay station includes a phase conjugate light generator including a signal light and excitation light supply section which supplies an input signal light from the sending station and an excitation light to a nonlinear optical medium and a signal light and phase conjugate light extraction section which extracts an output signal light and a phase conjugate light generated by the input signal light and the excitation light supplied to the nonlinear optical medium and a modulator which modulates the excitation light by monitor data specific to the relay station. The relay station transmits a modulated phase conjugate light including the monitor data to the receiving station (see, for example, Japanese Patent No. 3436310).
With WDM, however, there is a limit to accuracy in setting wavelength for a signal light or the resolution of an optical multiplexer and demultiplexer. As a result, it is difficult to realize high density multiplexing. Accordingly, there is a limit to the number of optical signals which can be multiplexed.